1. Field of the Invention
The present invention relates to a memory device, and to a method of operation of such a memory device in order to erase data stored within the memory device.
2. Description of the Prior Art
In a typical memory device comprising an array of memory cells arranged in rows and columns, data remanence may be observed since such a memory device often retains its last stored value even after the processing device using that memory device has no further need for that stored value, and hence the memory cells storing that value are available for overwriting with new data. Only once those memory cells have actually been overwritten with new data will the old data no longer be available in the memory device.
However, arranging the processing device to take positive steps to overwrite data within the memory device as soon as it has finished using that data has typically been considered to incur too much of a performance penalty, particularly where that data is stored within memory cells occupying multiple rows of the memory array, and accordingly a sequence of write operations would be required to overwrite the data one row at a time.
When the memory device is used to store sensitive data, it is hence more common to rely on software protection measures to ensure that that sensitive data is not accessed by applications that are not authorised to access it. However, it has been found that such software protection measures are prone to attack by hackers seeking to access that sensitive data.
For example, one known approach, often referred to as a cold boot attack, involves turning off the system incorporating the memory device and associated processing device, and then turning it on again. This allows the software protection measure to be circumvented. Under normal conditions, this would not be a problem, as removing the power supply to the memory device would typically result in the stored data being lost. However, it has been found that even after the power has been removed from the memory device, the information stored therein can remain for a prolonged period of time when special, external, conditions are applied, this providing another form of data remanence. One example of such a special external condition is to subject the memory device to a low temperature before removing the power. Under such conditions, once the power is restored, the software protection will have been circumvented, and the data may still be extracted from the memory device.
It would be desirable to provide an improved technique for preventing access to previously stored data in a memory device.